This invention concerns a method for manufacturing binder for lead storage battery glass mats.
In general, lead storage batteries, as shown in FIGS. 2 and 3, have PbO.sub.2 anode plates 3 and Pb cathode plates 4 installed at a specific distance from each other and separators composed of superimposed glass papers 5 and glass mats 6 inserted between the aforementioned electrodes plates 3 and 4; the whole is immersed in an electrolyte solution consisting of an H.sub.2 SO.sub.4 solution. 7 is a pole. This kind of glass mat 6 should be durable with respect to the electrolyte solution, in the first place, and it should also have a good permeability for the electrolyte solution and high mechanical strength. That is, in the process of assembling the lead storage battery, the separators with the glass mats 6 and the glass papers 5 superimposed on each other are driven in between the anodes 3 and the cathodes 4, and must have sufficient strength to withstand this driving operation and preserve the aformentioned electrodes; moreover, they should not be invaded by the electrolyte solution, and the electrolyte solution should be able to pass through them easily, from the point of view of producing electrical power. From these points of view, acrylate polymers, which have excellent acid resistance, are used as binders for binding the fiber structure of the aforementioned glass mats 6 and reinforcing them. In order to further increase the reinforcing effect of the aforementioned acrylate polymers, polymerizable unsaturated carboxylic acids such as acrylic acid or methacrylic acid and various kinds of thermo-crosslinking monomers can be copolymerized with them. For example, as such thermo-crosslinkable monomers, monomers with hydroxyl or glycidyl groups (Public Patent Bulletin No. 57-26546) or monomers with glycidyl or N-alkylol groups (Public Patent Disclosure Bulletin No. 61-288370), or organosilicon monomers (Public Patent Disclosure Bulletins Nos. 59-203738 and 61-155763) can be used. However, in acrylate copolymer binders in which these thermo-crosslinking binders are copolymerized, crosslinking reactions occur between the parts introduced from the thermo-crosslinking monomers and the parts introduced from the polymerizable unsaturated carboxylic acid, as time passes. If this phenomenon occurs before processing into the glass mat 6 is performed, there is the problem that the effect of reinforcing the glass mat 6 will be diminished.
The aforementioned lead storage battery glass mat binders require that the mechanical strength of the glass mats be reinforced, and also that the quantity of binder adhering to the glass mats be made small, so that the electrolyte solution will flow through them more easily. That is, in order to reduce the quantity adhering to the binder and produce a suitable mechanical strength, it is desirable for the binder to adhere uniformly not only to the surface of the glass mat but also inside it. From this point of view, the viscosity of the binder should be made high, and it has been proposed that the viscosity of the acrylic acid copolymer binder be raised by adding an alkali at the time it is used (Public Patent Bullentin No. 48-277093). However, even in such alkali-thickened binders, an acrylic acid copolymer, with a thermo-crosslinking monomer and a polymerizable unsaturated carboxylic acid copolymerized in, is used as the principal ingredient. In copolymers with such a structure, like the copolymers mentioned previously crosslinking reactions occur between the thermo-crosslinkable monomer parts and the carboxylic acid parts, and the ability to produce the viscosity obtained by thickening with an alkaline substance (abbreviated below as "alkali thickenability") and the property of tight adhesiveness to the glass fibers are greatly reduced over time. Consequently, in the aforementioned binders which use this principal ingredient, the aforementioned properties are also greatly reduced over time. Moreover, the proposal has also been made to incorporate organosilicon monomers with these binders and thus strengthen the adhesiveness with the glass fibers (Public Patent Disclosure Bulletin No. 59-203738). Although this binder has an increased adhesiveness to the glass fibers due to the aforementioned organosilicon monomers, the principal ingredient of the binder is an acrylic acid copolymer made by copolymerizing a thermo-crosslinking monomer and a polymerizable unsaturated carboxylic acid, and its alkali thickenability and adhesiveness to the glass fibers are still greatly reduced over time due to the occurrence of a crosslinking reaction. In order to correct this defect, a method has been proposed in which the reduction in the adhesiveness of the binder to the glass fibers over time is reversed by adding organosilicon compounds (Public Patent Disclosure Bulletin No. 61-288370). However, in this method, it is necessary to add the organosilicon monomer immediately before the binder is used, making the operation more complex, and in addition the reduction in alkali thickenability over time cannot be improved. Thus, up to now, glass mat binders have not existed which exhibit sufficient mechanical strength while reducing the quantity of binder adhering [to the glass mat], and the binder properties of which do not decrease over time.
This invention was made with these facts in mind. Its purpose is to provide a lead storage battery glass mat binder which exhibits sufficient mechanical strength even though the quantity adhering [to the glass mat] is small, and which does not suffer a decrease in its binder properties over time.